The prior art includes a number of patents teaching modular units held together with tongue and groove joints in which each tongue tapers to a larger size as the distance from the side of unit increases and the groove gets wider as its depth into its unit increases. The prior art using such mating tongue and groove joints includes: Brown U.S. Pat. No. 3,194,426, granted Jul. 13, 1965, Vom Stein et al. U. S. Pat. No. 3,763,980, granted Oct. 9, 1973, Nagata U. K. 1,539,870, published Feb. 7, 1979 and Moore U. S. Pat. No. 4,624,383, granted Nov. 25, 1986.